Polydisperse Versus Monodisperse Aerosols

Many of the basic principles of aerosol deposition theory have been validated with laboratory-generated monodisperse aerosols, which means that the particles are all the same size. A perfectly monodisperse aerosol has a geometric standard deviation (GSD) of 1.0, but an aerosol is said to be acceptably monodisperse if the GSD is less than 1.22 (1). 

In 1935, Findeisen published his Uber das Absetzen kleiner in der Luft sus-pendierter Teilchen in der mensliche Lunge bei der Atmung, which was the first attempt to calculate the deposition of aerosols (3). The Findeisen model included four mechanisms for deposition of particles. These were (1) impaction, (2) sedimentation, (3) Brownian diffusion, and (4) interception. This review focuses only on the first two mechanisms proposed by Findeisen, because Brownian diffusion affects particles 1 jm these are nsnally too small for therapeutic purposes and interception is normally insignificant except for elongated particles such as fibers. For more information abont diffusion and interception, we refer the reader to Chap. 2. 

Electrostatic precipitation is another mechanism for particle deposition it is important when aerosols are administered from holding chambers or spacers. We refer the reader to Chap. 12 for more information about electrostatic charge 

Marcel Dekker, Inc. 270 Madison Avenue. New York, New York 10016 

The predominant mechanism for deposition of particles 3 pm is inertial impaction. Inertial impaction occurs primarily in the oropharynx, trachea, and larger conducting airways. This is because these particles cannot follow the sudden airflow changes in these airways, and they collide with the airway wall. 

---

Most aerosols are polydisperse when formed, some more than others. For example, an examination of sawdust would reveal particles of various sizes, as would that of any material formed by attrition. Since raindrops could grow by condensation or by a series of collisions with other drops, they would also be expected to be polydisperse. In fact, monodisperse aerosols are very rare in nature, and when they do appear, generally they do not last very long. Some high-altitude clouds are monodisperse, as are some materials formed by condensation. Sometimes it is satisfactory to represent all the particle sizes by only a single size. Other times more information is needed about the distribution of all particle sizes. Of course, a simple plot of particle frequency versus size gives a picture of the sizes present in the aerosol, but this may not be enough for a complete quantitative analysis. 

---